Technomotive Presents

The Diagnostic Port

Question for the week of September 15th, 1997

I read in the shop manual that the engine computer remembers more than just engine error codes. What else does it remember?

The ECU "learns" about your engine as you drive the car. The "learning" is actually a process that the ECU uses to track the tolerance changes of the sensors and actuators on the engine. Two examples of these values are the position of the idle-air bypass valve (automatic choke) at idle with the A/C on, and the trim gain on the injectors. The ECU stores these "learned" values in battery backed-up RAM so that it doesn't have to start from scratch the next time you turn your engine over.

I've heard that after I make a change to my car, I have to unplug the engine computer so that it will learn about my new equipment. Is that true?

There is quite a bit of mythology involving resetting the ECU. Most of it is flat wrong. The ECU is always tracking the gradual changes in the engine sensors and actuators. The ECU never gets satisfied with a setting and quits updating the trim values. It will discover whatever change you made to your engine and make the appropriate changes. In fact, unplugging the ECU will most likely increase the learning curve of the new part.

One good reason to reset the ECU is to clear a CHECK ENGINE light or error code after fixing the problem that caused the error.

How do I reset the engine computer?

Resetting the ECU is actually quite easy on both the first and second generation DSMs. Make certain the key isn't in the ignition. Locate the fuse panel on the left side of the driver's footwell. On the first generation, pull the 10A fuse in the lower right-hand corner. On the second generation, pull the 10A fuse on the right-hand side of the middle row. Wait 15 seconds before replacing the fuse - that should drain any stored charge in the ECU. Your ECU is now reset.

I reset the ECU, and now the car runs rough.

The engine may run rough or idle poorly right after ECU reset. The ECU now has default numbers in its learn memory - it has to adjust them to match the tune of your engine and its sensors and actuators. It might take a day or two of driving in traffic before the engine will settle down again. Now you know why the ECU remembers these values in the first place!

Past Diagnostic Port issues available.

Do you have a question about engine computers? Go ahead and ask it!

©1997 Technomotive